feat(nav): mobile bottom tab bar and ≥44px logo tap target

MOB-01: At ≤640px the top inline nav is hidden and replaced by a
fixed bottom tab bar (Search / Compare / Rankings / Admissions) with
icon + label, 56px tap targets, env(safe-area-inset-bottom) padding,
and the Compare count rendered as a badge on the icon. Eliminates
horizontal page overflow on every route (docW was 401–429 at vw 390;
now docW === vw).

MOB-02: Logo link gains a padded hit area so the touch target is
≥44×44 (was 36×36), without resizing the visual mark.

ComparisonToast lifted above the new bottom bar on mobile so the two
do not stack on top of each other. body gets a bottom padding equal
to the bar height + safe-area inset so page content is never hidden.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

nextjs-app/app/globals.css

@@ -95,6 +95,13 @@ body {
     min-height: 100vh;
 }
 
+/* Reserve space for the fixed mobile bottom tab bar (56px + safe-area inset). */
+@media (max-width: 640px) {
+    body {
+        padding-bottom: calc(56px + env(safe-area-inset-bottom, 0px));
+    }
+}
+
 /* Skip link — visible only on focus for keyboard users */
 .skip-link {
     position: absolute;